Composition and method for forming amorphous chalcogenide films from solution

ABSTRACT

Amorphous chalcogenide films substantially free from particulate and microstructure are formed on a substrate from solution. The solution contains a glass-forming chalcogenide compound dissolved in a non-aqueous vaporizable solvent, such as a low molecular weight amine, and is substantially free from particulate or crystallizable material. Film formation is effected by coating the solution onto the substrate in a non-vacuumized environment, and thereafter evaporating the solvent from the coating. The procedure is particularly useful in forming amorphous chalcogenide resists for photolithographic applications.

This is a divisional application of U.S. patent application Ser. No.377,222, filed on May 11, 1982, which is now U.S. Pat. No. 4,439,464.

BACKGROUND OF THE INVENTION

This invention relates to amorphous chalcogenide films for use asphotolithographic resists and the like, and, more particularly, to novelcompositions and procedures for fabricating such films on a substrate.

Amorphous chalcogenide thin films have recently found utility in anumber of applications in the electronics industry. For example, thechalcogenide glasses, such as As₂ S₃, As₂ Se₃, Se--Ge, and other binaryand ternary glassy compounds of sulphur, selenium and tellurium, arebecoming increasingly important as radiation-sensitive resist materialsin the photolithographic fabrication of integrated circuits. Theamorphous chalcogenide thin films have also been investigated for use asoptical information storage media, as xerographic films, as films forovonic memories and switches, and as anti-reflection coatings andUV-absorbing coatings for lenses and other optical systems.

In all of the various aforementioned applications, fabrication of theamorphous chalcogenide thin films on a substrate has typically beeneffected by vacuum deposition techniques, i.e., either by thermalevaporation or by sputtering. In addition to the relatively high cost ofthese techniques, vacuum deposited films generally suffer from a lack ofcontrollable reproducibility in their stoichiometry, morphology andmicrostructure. Moreover, solution coating techniques have heretoforebeen generally avoided due to the fact that the only previously knownsolvents for the chalcogenides, i.e., aqueous alkalies and acids, arereactive with the chalcogenides and convert them to crystallizablecompounds.

SUMMARY OF THE INVENTION

It is, accordingly, a primary object of the present invention to providean improved method for fabricating amorphous chalcogenide films onsubstrates which is more convenient and economical to carry out than thecurrently employed vacuum deposition techniques.

Another object of the invention is to provide an improved method forfabricating amorphous chalcogenide films on substrates which enables thefilms to be produced with controllable reproducibility and uniformitywith respect to their stoichiometry, morphology and microstructure.

A further object of the invention is to provide an improved method forfabricating amorphous chalcogenide films on substrates employingsolution coating techniques.

The above and other objects are achieved in accordance with the presentinvention by providing amorphous chalcogenide film-forming compositionswhich are substantially free from particulate or crystallizable materialand which comprise a glass-forming chalcogenide compound dissolved in anon-aqueous vaporizable solvent. The solutions of the present inventionmay be coated onto a substrate in a non-vacuumized environment to form,upon evaporation of the solvent from the coating, an amorphouschalcogenide film substantially free from particulate andmicrostructure. Such films can be formed in accordance with the presentinvention with controllable reproducibility and uniformity with respectto their stoichiometry and morphology.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is based upon the discovery that the glass-formingchalcogenide compounds can be dissolved in certain solvents without anysubstantial conversion to crystallizable compounds so as to formsolutions which are substantially free from particulate orcrystallizable material and which can be coated onto suitable substratesby convention solution coating techniques to form, upon evaporation ofthe solvent, amorphous chalcogenide films which are substantially freefrom particulate and microstructure and substantially uniform inthickness and composition. All of these solvents are non-aqueous andvaporizable. A number of organic solvents, particularly low molecularweight amines, such as lower alkyl amines and lower alkylene diamines,have been found to be particularly suitable. Specific examples of suchlow molecular weight amines include ethylene diamine, n-propylamine,diethylamine, triethylamine, and the like. Carbon disulfide, a solventfor the elemental chalcogens, was not found to be a suitable solvent forthe glass-forming chalcogenide compounds.

The foregoing solvents are capable of suitably dissolving a wide varietyof glass-forming chalcogenide compounds so as to form amorphouschalcogenide film-forming compositions. By way of example, the dissolvedchalcogenide compound may be any of the various Se--Ge glass-formingcompounds of varying stoichiometry, as set forth in U.S. Pat. No.4,127,414, issued Nov. 28, 1978, incorporated herein by reference.Alternatively, the dissolved chalcogenide compound may be an arsenicchalcogenide represented by the formula As₂ Z_(2+x), where Z is S, Se orTe, and x is an integer of from 0 to 3. Representative arsenicchalcogenide compounds include As₂ S₃, As₂ S₂, As₂ Se₃, and As₂ Te₃.

Solutions of other arsenic chalcogenides encompassed by the aboveformula where x is greater than 1, can be readily prepared in accordancewith the present invention with controlled stoichiometry. This procedurecomprises dissolving in the appropriate solvent the arsenictrichalcogenide, i.e., As₂ Z₃, and partially oxidizing or hydrolyzingthe dissolved As₂ Z₃ to an extent sufficient to convert it to thecorresponding arsenic chalcogenide compound having the desiredstoichiometry and an arsenic-containing precipitate. Such conversion cansuitably be effected, for example, by including a small amount of water,in the appropriately controlled concentration, in the solvent employedfor dissolving the As₂ Z₃. The arsenic-containing precipitate thereaftercan be readily separated out, so as to recover the resulting supernatantcontaining dissolved therein the arsenic chalcogenide of the desiredstoichiometry.

The concentration of glass-forming chalcogenide compound present in theamorphous chalcogenide film-forming compositions of the presentinvention may suitably be varied over a wide range, from very dilute upto saturated solutions, which will generally depend upon the desiredthickness of the film to be deposited. For film thicknesses in the rangeof from about 0.2 to about 2 microns, for example, particularly suitablesolutions are those in which the chalcogenide compound is present in anamount within the range of from about 1 to about 25% by weight.

The amorphous chalcogenide film-forming compositions of the presentinvention may suitably be coated onto a substrate, in a non-vacuumizedenvironment, by any of the conventional solution coating techniques wellknown in the art, for example, by spin-on, dip-coat or spray-coattechniques. A spin-on coating technique has been found to beparticularly suitable for thin film fabrication. Following the coatingstep, solvent evaporation from the coating may be effected either atroom temperature or at elevated temperatures, e.g., up to approximately100° C., depending upon the volatility of the solvent employed. By wayof example, with ethylene diamine (having a boiling point of 116° C.) asthe solvent, vacuum baking at approximately 100° C. for about 30 minuteshas been found to be adequate.

The coating and evaporation steps are preferably carried out in anon-oxidizing and non-hydrolyzing environment so as to avoid conversionof the chalcogenide to a particulate or crystallizable material whichwould adversely affect the film properties. This may suitably beaccomplished, for example, by directing a dry nitrogen spray onto thesubstrate surface during the coating operation, which also effectivelyaids solvent evaporation. In following this procedure, care should beexercised in avoiding excessive nitrogen flow so as not to producesurface rippling in the film.

The compositions and methods of the present invention may suitably beemployed for fabricating amorphous chalcogenide films on a substrate foruse in a wide variety of applications, such as those enumerated above.The invention is particularly useful in conveniently and economicallyforming radiation-sensitive amorphous chalcogenide resist materials fordelineating a pattern on a substrate in the photolithographicfabrication of integrated circuits. Thus, after forming the amorphouschalcogenide thin film on the substrate by means of the solution coatingtechniques of the present invention, a silver-containing overlayercapable of photodoping the amorphous chalcogenide film for enhancementof its resist properties may be coated onto the amorphous chalcogenidefilm, also by solution coating techniques, e.g., by dipping into anaqueous solution of silver nitrate, or by spin-on of a thin film ofsilver halide or silver chalcogenide from the appropriate solvent.Preferably, both the amorphous chalcogenide film and thesilver-containing overlayer are applied by means of spin-on coatingtechniques, thereby providing an entirely spin-on inorganic resistsystem. The resulting resist system may then be used in the conventionalmanner for carrying out the remainder of the photolithographicprocedure, i.e., by exposing the resist system through a mask to asuitable source of actinic radiation (e.g., UV light, electron beams,ion beams, x-rays, or the like), dipping the exposed system into 25%aqua-regia to remove unreacted silver-containing medium, and developingin aqueous NaOH or by plasma processing.

The invention is further illustrated by way of the following examples.

EXAMPLE 1

Solutions of arsenic trisulfide were prepared from As₂ S₃ glass powder(Servofax) and either anhydrous ethylene diamine (Analar grade reagent)or anhydrous n-propylamine. The solutions contained about 0.3 g As₂ S₃per ml of solvent, corresponding to approximately 25% solids. Theresulting mixtures were yellow-orange in color and moderately viscous.There was no gross precipitation from these solutions, although anyexposure to water either during storing or subsequent thin filmfabriction caused a precipitate.

Thin films of chalcogenide glass were spun onto either 21/2 inch×21/2inch glass plates for examination of homogeneity, approximately 1/2inch×1/2 plates for UV spectrophotometry or polished high resistivity 1inch or 2 inch diameter silicon wafers for IR spectrophotometry andX-ray diffraction as follows. Substrates were placed on the spinner. Thesolution of the chalcogenide was centrifuged at 3,000 rpm and/orfiltered to 0.5 μm, and then pipetted onto the substrate surface andallowed to cover approximately 75%. Spinning was at 3,000 rpm, for 30seconds. A dry nitrogen spray was directed onto the substrate surfaceduring spinning (5 mm nozzle placed 6 inches above the substrate: 35 psiN₂ pressure). This procedure aided solvent evaporation and excluded H₂ Oand O₂ during film deposition. The films were vacuum baked atapproximately 100° C. for 30 minutes, and stored in a dessicator priorto testing.

The resulting films were 1.8 microns thick on the 21/2 inch square glassplate, with less than 5% thickness variation over the entire substrate,as measured by optical interference. Visual examination showed no grossfeatures, while optical and scanning electron microscopy indicated amicrostructure-free material down to less than 0.1 micron. The filmswere amorphous, as shown by X-ray diffraction. IR and UVspectrophotometry indicated that the films showed absorption featuressimilar to that of bulk As₂ S₃ samples.

EXAMPLE 2

The procedure of Example 1 was repeated, but employing as the solventethylene diamine containing 8% H₂ O. The resulting solution formed ayellow-white precipitate. X-ray powder diffraction of the precipitateindicated a complex mixture of oxy and hydroxy compounds of arsenic andorganoarsenic compounds. After a few hous, the precipitation ceased, andthe clear solutions were quite stable for prolonged periods of storagein a stoppered bottle. The resulting film showed properties similar tothose as described in Example 1. However, the visible-UV transmissionspectra indicated an absorption edge close to bulk As₂ S₅, rather thanthe starting As₂ S₃.

EXAMPLE 3

An arsenic trisulfide film deposited from ethylene diamine solution asdescribed in Example 1 was sensitized by dipping into 10⁻³ M AgNO₃, andemployed as a photolithographic resist material. This resist required 3J cm⁻² UV exposure, which is comparable with the fastest vacuumevaporated As₂ S₃ resists reported in the literature. While plasmaetching speed of the unexposed portions of the resist were somewhatslower than the optimum plasma etching speeds of unexposed vacuumevaporated As₂ S₃ films reported in the literature, 1-2 micron lines andspaces were satisfactorily printed with edge sharpening effects.

I claim:
 1. In a photolithographic process comprising the steps offorming a radiation-sensitive amorphous chalcogenide film on a substrateand employing said film as a resist in delineating a pattern on saidsubstrate, the improvement wherein said film formation is effected bythe steps comprising:(a) coating onto said substrate in a non-vacuumizedenvironment a solution containing a glass-forming chalcogenide compounddissolved in a non-aqueous vaporizable solvent, said solution beingsubstantially free from particulate or crystallizable material; and (b)evaporating the solvent from the coating so as to form an amorphouschalcogenide film substantially free from particulate andmicrostructure.
 2. The process of claim 1, wherein said solvent is anorganic solvent.
 3. The process of claim 2, wherein said solvent is alow molecular weight amine.
 4. The process of claim 3, wherein saidsolvent is a lower alkyl amine or a lower alkylene diamine.
 5. Theprocess of claim 1, wherein said chalcogenide compound is Se--Ge or As₂Z_(2+x), where Z is S, Se or Te, and x is an integer of from 0 to
 3. 6.The process of claim 1, wherein said coating and evaporation steps arecarried out in a non-oxidizing and non-hydrolyzing environment.
 7. Theprocess of claim 1, wherein said coating step is effected by means of aspin-on coating technique.
 8. The process of claim 7, wherein asilver-containing overlayer capable of photodoping said amorphouschalcogenide film for enhancement of its resist properties is coatedonto said amorphous chalcogenide film by means of a spin-on coatingtechnique.